This application makes reference to and claims all benefits accruing under 35 U.S.C. Section 119 from an application for METHOD FOR CONTROLLING THE FORWARD CALLS AND DETERMINING THE TRANSMISSION POWER IN A BASE STATION OF A CELLULAR RADIO TELECOMMUNICATION SYSTEM filed earlier in the Korean Industrial Property Office on Apr. 3, 1999 and there duly assigned Serial No. 11719/1999.
1. Field of the Invention
The present invention relates to a cellular radio telecommunication system, and more particularly to a method for controlling forward calls with a predetermined transmission power.
2. Description of the related art
In a cellular radio communication system, a geographical area, e.g., a metropolitan area, is divided into several smaller, contiguous radio coverage areas known as xe2x80x9ccells.xe2x80x9d Each cell is served by a fixed radio station known as xe2x80x9cbase station.xe2x80x9d The base stations are connected to and controlled by a mobile service switching center (MSC), which communicates voice and/or data signals with the mobile phones. The forward channel (from the base station to the mobile phone) is divided into a traffic channel for transmitting voice data and an overhead channel for transmitting control data.
As shown in FIG. 1, a conventional base station (BS) includes an Intermediate Frequency (IF) modulator 110 for modulating the transmitted signal of each channel into an IF signal; a frequency-up converter 120 for converting the IF signal into a radio frequency (RF) signal; a radio frequency unit (RFU) 130 for amplifying the RF signal and for eliminating unwanted waves; an antenna 140 for transmitting the RF signal; and, a base station control processor (BCP) 150 for controlling the overall function of the base station. The base station (BS) provides each call with a traffic channel to exchange voice and data signals.
The forward overhead channel serves to transmit the messages necessary for preparing the communication channel between the mobile station (MS) and the base station (BS) using almost a constant output power. However, the output power of the entire traffic channels of the BS relates to the number of calls processed for transmitting different voice messages. The number of calls that can be simultaneously processed by the BS is determined by the BS control processor 150.
Each traffic channel of the base station is assigned to a specific transmission power. Thus, if the BS simultaneously receives an excessive number of calls, its total transmission power exceeds the proper limit of the system, causing the high power amplifier (HPA) of the RFU 130 to malfunction. Namely, the overflow causes nonlinear distortion in the HPA that degrades the communication quality of the BS and causes call interruption.
In order to resolve above problem, one conventional method measures the total transmission power of the BS by detecting the signal just prior to transmission through the antenna 140 with a BS test unit (BTU) 160. Then, the measured power is compared to a predetermined threshold value in determining whether to limit the calls. If the total transmission power is higher than the predetermined threshold value, the BS limits the calls generated thereafter. Thus, a reliable performance of the BTU 160 can yield an accurate determination in limiting the correct number of calls. For example, the forward measuring error of the BTU 160 may result in an inaccurate measurement of the transmission power that is lower than the actual value, causing inaccurate determination in limiting the calls by the base station. Also, as the BTU 160 successively measures the transmission power for each of the frequency channels provided in the base station (BS), the time required to measure the total transmission power is prolonged, further causing the problem of determining the accurate number of calls to limit during the system overload.
Another conventional method employs a call admission control (CAC) method to predetermine the total transmission power of the BS based on the transmission power of each channel generated from the IF modulator 110 in order to eliminate the measuring error of the BTU 160. In the transmission path of the BS arranged as shown in FIG. 1, the power level generated from the IF modulator 110 is determined by the digital gain (D/G) of each channel, so that the total transmission power of the BS can be determined by summing up all digital gain (D/G) for each channel. As the D/G is assigned to each call requested through the BS, the power obtained by the D/G of the channel represents the power assigned to the corresponding call in the IF modulator 110. However, this method also falls short of determining an accurate transmission power value due to the error associated with the BTU. That is, there is till error associated with the BTU 160 as the output power of the IF modulator 110 obtained by the D/G must flow through the frequency-up converter 120 and the RFU 130, causing some error in measuring the total transmission power by the BTU 160.
It is an object of the present invention to provide a method for controlling the forward calls according to the transmission power which is predetermined based on the measured value of the BTU and the D/G of the IF modulator in a cellular radio system.
It is another object of the present invention to provide a method for controlling the forward calls according to a predetermined transmission power which compensates the error occurring in the radio frequency path and which is stored in the base station testing unit (BTU) in a cellular radio system.
According to the embodiment of the present invention, a method for controlling the forward calls in a cellular radio telecommunication system includes the steps of: reading the transmission power of the BS measured by the BTU; comparing the transmission power with a first predetermined threshold value; storing the transmission power if the read transmission power is higher than the first predetermined threshold value; calculating the digital gain power using the digital gain assigned to a call to the BS; calculating the total transmission power of the BS according to the call by adding the stored transmission power to the calculated digital gain power; and, limiting the call in response to the determination of comparing the total transmission power with a second predetermined threshold value.
According to another aspect of the present invention, a method for controlling the forward calls in a cellular radio telecommunication system, comprising the steps of: reading the transmission power error occurring in the radio frequency path from the BTU of the BS; calculating the digital gain power using the digital gain assigned to a call to the BS; calculating the total transmission power of the BS according to the call by adding the read transmission power to the calculated digital gain power; and, limiting the call in response to the determination of comparing the total transmission power with a predetermined threshold value.
In order to control the forward calls of the BS in a cellular system, the first aspect of the present invention is to store the transmission power if it exceeds the first predetermined threshold value, then the stored transmission power is added to the digital gain power calculated by the D/G assigned to a newly generated call in order to produce the total transmission power of the BS.
A second aspect of the present invention is to store the error power occurring in the radio frequency path (RFU) into the BTU and to add the stored error power to the digital gain power calculated by the D/G assigned to a newly generated call to produce the total transmission power of the BS. To this end, the BTU first detects the signal before flowing into the radio frequency path, and then detects the signal flowing out of the radio frequency path or flowing just before entering the transmission antenna. Thus, the two detected signals are compared to determine the error power.
The present invention will now be described more specifically with reference to the drawings attached as a way of example.